1. Field of the Invention
This invention relates to the field of highly purified solids. This invention more particularly relates to a vapor-liquid-solid (VLS) process and apparatus for preparing highly purified solids, such as tellurium and cadmium. This invention especially relates to an improved process and apparatus for preparing highly purified solids of tellurium or cadmium by the combined effect of distillation and normal freezing wherein the rate of condensation is substantially greater than the rate of crystallization so that the excess liquid continually washes the growing solid removing the impurities rejected by solidification.
2. Description of the Prior Art
Vacuum distillation of metals is a well known process. See (Ultrapurification of Semiconductor Materials, Proc. Conf. on Ultrapurification of Semicond. Mat., Boston, Mass., April 1961, M.S. Brooks and J.K. Kennedy, Eds. (MacMillan, New York 1962); Harman et al., J. Phys. Chem. Solids, 2 181 (1957)). The use of vapor transport to move material into a process tube is also known (Lawson et al., 9 J. Phys. Chem. Solids, 325 (1959); Dziuba, 26 Acta Phys. Polonica, 897 (1964)). Faktor and Garrett in "Growth of Crystals from the Vapour", (Chapman and Hall, London 1974) discussed the effect of small amounts of inert gas on vapor transport. Russell and Woods (46 J. Crys. Growth 323 (1979)) demonstrated that extraneous gas evolving from a sealed system is difficult to control and requires clever manipulation for detection and measurement. Some investigators who experienced low vapor flux in a VLS system assumed that the solid growth was limited by the processes occurring at the liquid-solid interface. (Andryushenko et al., 15 Izv. Akad. Nauk SSSR, Neorq. Mater 1573 (1979).)
It is an object of this invention to prepare highly purified solids.
It is another object of this invention to improve VLS systems for preparing purified solids.
It is a further object of this invention to prepare highly purified tellurium and cadmium in an improved VLS system.
The achievement of these and other objects will be apparent from the following description of the subject invention.